https://www.lemonparty.org was the original one, but it seems to have changed into some shitty page full of pop-ups and spam... R.I.P
Several years back on TNB one of the regulars there owned GILF and it got some pretty solid traffic although if memory serves correctly, it was for grandmas not granpas.
There have been several programs launch grandpa/grandson type sites over the years, mostly originating around Eastern Europe and Russia, but some of the US based programs had a few too, so there is obviously a market for this type of content.